Various types of magnetic field sensing elements or transducers are known, including Hall effect and magnetoresistive elements. Generally, sensors or detectors including magnetic field sensing elements provide an electrical signal representative of the sensed magnetic field and, in the presence of a moving ferromagnetic target object, the magnetic field signal is indicative of the shape or profile of the target object.
Magnetic field sensors are often used to detect gear features, such as gear teeth and/or gear slots. A magnetic field sensor in this application is commonly referred to as a “gear tooth” sensor. Gear tooth sensors are used in automotive applications to provide information to an engine control unit for ignition timing control, fuel management, and other operations.
In one type of gear tooth detector, the magnetic field signal is compared to a switching threshold signal to provide the detector output signal at a first binary level when the magnetic field signal is less than the switching threshold signal and at a second binary level when the magnetic field signal is less than the switching threshold signal. With this arrangement, the detector output signal transitions at the same point on each gear tooth. One such detector is described in U.S. Pat. No. 6,525,531 in which a Positive Digital-to-Analog Converter (PDAC) and a Negative Digital-to-Analog Converter (NDAC) track the positive and negative peaks of the magnetic field signal, respectively, for use in generating the switching threshold signal. This type of detector, in which the switching threshold signal is equal to a percentage of the peak-to-peak magnetic field signal, is sometime referred to as a peak-to-peak percentage detector.
Preferably, each detection of a particular feature of the passing magnetic article occurs at the same point on the magnetic field signal. A variation in the phase of detections of the same feature on different revolutions of the magnetic article or of different features on the same revolution is referred to as an error or jump in the phase of the detector output signal and can disadvantageously affect control units, such as engine control units in automotive applications, relying on the detector output signal.
Detection accuracy is adversely affected if the magnetic field signal experiences a change in magnetic offset. An offset shift can occur as a result of a changing spacing (or airgap) between the magnetic article and the magnetic field transducer, mechanical stresses, magnetic interference from nearby components or pollutants, and/or temperature variations. Changes in the amplitude or offset of the magnetic field signal degrade the accuracy of the magnetic article detection since the magnetic field signal is not centered within the dynamic range of the detector. In particular, accuracy is degraded when the magnetic field signal shifts so that the PDAC and NDAC signals no longer hold the true magnetic peak/valley. Since the switching threshold is a percentage of the difference between the PDAC and NDAC signals, the switching threshold will not align with the desired percentage point on the magnetic signal.
One technique for canceling an offset shift experienced by the magnetic field signal is to detect the presence of an offset shift greater than a predetermined amount and to adjust the level of the magnetic field signal accordingly, so as to cause the magnetic field signal to remain within the dynamic range of the detection and preferably, substantially centered within the dynamic range of the detector. However, under certain operating conditions, a significantly offset magnetic field signal having a relatively small peak-to-peak voltage may not cross the switching threshold signal, thereby causing a failure of the detector output signal to switch.